Following a cerebrovascular accident (CVA), individuals initially tend to lean toward the side of the hemiparesis. A long-term effect of this condition has been termed Pusher Syndrome. It has been shown that there is a severely disturbed perception of body orientation unrelated to vestibular dysfunction (e.g. a sensorimotor mismatch without vestibular dysfunction affecting position sense). It has been suggested that it is caused by a disturbed representation of body-space orientation (e.g. spatial organization to maintain upright body position against gravity), in turn affecting leg-trunk orientation and causing a lean into the affected side and a ‘push’ away from the functional side. However, within the first few weeks many will develop compensations causing a tendency to lean away from the hemiparetic side. The assumption is that this phenomenon has a neurological cause related to the weakness of the paretic side. The significance of this trend is that the lean into the affected side (Pusher's Syndrome) and then the developed compensatory lean away from the affected side is due to a reorganization of information from sensorimotor systems in context with weakness from the paretic side.
In addition to leaning away from the paretic side following a CVA, it is also common to have a neglect of one's visuo-spatial awareness on the affected side. It has been reported that 60% of CVA patients had a unilateral neglect. Furthermore, spatial neglect will cause a postural imbalance, thereby causing weight-bearing asymmetry (e.g. inappropriate body-space orientation causing a lateral shift in perceived concept of upright posture against gravity). When persons exhibit neglect, not only do they miss seeing objects in the neglected field, but they will often twist and/or lean away from the affected visual field. The twisting or turning of their body will compromise the ability to maintain equal weight displacement between the lateral components of their body. Some individuals will torque their bodies to the extent that they are looking and turning their bodies 90 degrees or greater away from their centre or midline. This deviation in body position is a phenomenon that has been interpreted related to a lack of attentive awareness regarding a lateral aspect of the visual field. However, consideration that this phenomenon is due to mismatch between the spatial component of the visual process and sensorimotor information may offer greater insight as to its cause.
Rehabilitation following a CVA requires extensive physical and occupational therapy in an attempt to facilitate proprioceptive and spatial awareness on the affected side, through weight-bearing, to improve posture and balance. It has further been reported that of patients receiving appropriate rehabilitative care, 20% had an increased length of stay related to CVA associated perceptual spatial problems.
Prisms have been demonstrated to have a beneficial effect to improve visually guided action and perception as well as to affect-posture among patients with CVAs. has been. demonstrated improvement in daily life activities, such as postural balance and spatial orientation through the use of prism adaptation (e.g. the use of prisms to establish visual and sensorimotor awareness of neglected field). It has been also found that improvement in visuo-manual adaptation with prisms occurs, thereby reducing visuo-spatial neglect. This further suggests that the prisms may activate brain functions related to multi-sensory integration.
A prism is a wedge of optical media typically made from plastic, glass, or polycarbonate Prisms are traditionally prescribed in glasses to compensate for a deviation in eye alignment. However, yoked prisms, as used in this study are not for compensation but instead used to affect position sense and orientation to body space. A prism is shaped with an apex at the thin end and a base at the thick end of the prism. The angle of degree deviates a beam of light such that the image of an object will be shifted a centimeter (cm) per dioptre of prism at a distance of 1 meter (m). For example, five dioptres of prism deviates an image 5 cm at a 1 m distance. A prism shifts an image due to properties that compress space in one direction and expand space in another direction. Yoked prisms expand and compress space equally for both eyes. The expansion and compression of space through the use of yoked prisms becomes the rehabilitative utility to neutralize the sensorimotor distortion caused by mismatch between the affected (paretic) side and the non-affected side.